How do you make sure that the light will be hanging in the right position in relation to the tank? I'm afraid to drill holes in the ceiling and find out that the light hangs a little bit off from the middle of the tank.
I found my tape measure, an old level, and a pencil did the job quite well. I ended up marking the wall up a few times to get it just right and level.
If you are mounting it on the ceiling, Nikki's idea is probably the simplest.